My first comment is that the movement of plant communities is part of the climate system, and is not, separate from “climate change“.

The LA Times article starts with the header

“A study in the journal Science challenges assumptions that climate change and rising temperatures would send vegetation to higher elevations. A rise in precipitation could be the cause.”

The text starts with

“Predictions that climate change will drive trees and plants uphill, potentially slashing their range to perilous levels, may be wrong, suggests a new study that found vegetation in California actually crept downhill during the 20th century.”

Other excerpts read

“But comparing data from the early and late 20th century, authors of the Science paper found that despite warming, many plant species in California mountain ranges are growing at lower elevations than they did 80 years ago. The scientists attributed the shift to a wetter climate in Central and Northern California, which offset the effect of higher temperatures.

The lesson, said coauthor John Abatzoglou, a University of Idaho assistant geography professor, is that “we’d be remiss if we just focus on temperature,” in forecasting the influence of climate change on plant life. “This might mean species extinction rates may not be as dire as predicted.”

Millar’s research in the Eastern Sierra and the Great Basin has also found that tree lines are moving down rather than up, although for slightly different reasons. They are shifting down slope into drainages that are cooler, and coincidentally, moister.

“Uphill shifts of species’ distributions in response to historical warming are well documented, which leads to widespread expectations of continued uphill shifts under future warming. Conversely, downhill shifts are often considered anomalous and unrelated to climate change. By comparing the altitudinal distributions of 64 plant species between the 1930s and the present day within California, we show that climate changes have resulted in a significant downward shift in species’ optimum elevations. This downhill shift is counter to what would be expected given 20th century warming, but is readily explained by species’ niche tracking of regional changes in climatic water balance rather than temperature. Similar downhill shifts can be expected to occur where future climate change scenarios project increases in water availability that outpace evaporative demand.”

we also found that vegetation was moving downslope {in the case we studied it was due to moister and cooler conditions which we attributed to added atmospheric water vapor from irrigation along the Colorado Front Range. The abstract in our paper is [highlight added]

“We present evidence that land use practices in the plains of Colorado influence regional climate and vegetation in adjacent natural areas in the Rocky Mountains in predictable ways. Mesoscale climate model simulations using the Colorado State University Regional Atmospheric Modelling System (RAMS) projected that modifications to natural vegetation in the plains, primarily due to agriculture and urbanization, could produce lower summer temperatures in the mountains. We corroborate the RAMS simulations with three independent sets of data: (i) climate records from 16 weather stations, which showed significant trends of decreasing July temperatures in recent decades; (ii) the distribution of seedlings of five dominant conifer species in Rocky Mountain National Park, Colorado, which suggested that cooler, wetter conditions occurred over roughly the same time period; and (iii) increased stream flow, normalized for changes in precipitation, during the summer months in four river basins, which also indicates cooler summer temperatures and lower transpiration at landscape scales. Combined, the mesoscale atmospheric/land-surface model, short-term trends in regional temperatures, forest distribution changes, and hydrology data indicate that the effects of land use practices on regional climate may overshadow larger-scale temperature changes commonly associated with observed increases in CO2 and other greenhouse gases.”

The new research corroborates our finding of the complexity of the climate system which was inadequately assessed in the 2007 IPCC reports.